Lubricating oil for use in a wet clutch of an automatic transmission, construction or agricultural machinery, etc., is required to have excellent thermal-oxidative stability, wear prevention properties and frictional characteristics suitable for a wet clutch. Lubricating oil standards for this use include the DEXRON III Standards by General Motors Corp. and MERCON Standards by the Ford Motor Co.
Lubricating oil meeting these standards comprises a base oil (such as a mineral oil and a synthetic oil) and various additives (such as antioxidants, detergent-dispersants, wear preventives, rust preventives, sequestering agents, friction coefficient modifiers, defoaming agents, colorants, seal swelling agents and viscosity index improvers). The lubricating oil generally contains a zinc type antioxidant-extreme pressure additive-wear preventive, such as a zinc dialkyldithiophosphate, in order to improve thermal-oxidative stability or wear prevention properties, or a phosphorus type extreme pressure additive-wear preventive, such as a phosphoric ester and a phosphorous ester, in order to enhance wear prevention properties.
In recent years, a lock-up clutch effective in improving fuel consumption has been incorporated into the automatic transmission of many automobiles. A lock-up clutch is used in a torque converter. However, because a conventional lock-up mechanism is not operated in a low speed region but only in a high speed region, torque transmission through the torque converter has been accompanied by a power transmission loss between the input rev count of the transmission and the output rev count of the engine in a low speed region, for example, at the time of starting a car. This has caused an increase in fuel consumption. For the purpose of reducing the power transmission loss and thereby improving fuel consumption, a slip control system has recently been introduced in which the lock-up mechanism works in a low speed region as well.
The problem is that the car body shudders as the slip controlled lock-up mechanism is engaged. Shuddering is liable to occur particularly when the coefficient of friction is reduced with an increase in slip speed. Therefore, there has been a demand for frictional characteristics wherein the coefficient of friction increases with an increase in slip speed.
However, because of the difficulty in obtaining satisfactory initial frictional characteristics, it is likely that new cars will encounter the problem of shuddering. In such a case, it has been a practice to previously impart sufficient slides to the lock-up clutch friction materials before delivery.
The automatic transmission of up-to-date automobiles must be light and compact for improving fuel consumption, but this tendency is accompanied by an increase in heat load. On the other hand, a torque converter lock-up clutch equipped with a slip control mechanism has been increasingly adopted for the same purpose.